Recently, the technological development of a liquid crystal display element for electronic paper that can maintain its display state even if there is not power and can rewrite data with low power has become active. One of representative liquid crystal display elements is, for example, a liquid crystal display element that uses cholesteric liquid crystal.
FIG. 33 is a diagram illustrating a molecular structure example of cholesteric liquid crystal. Cholesteric liquid crystal is made by adding addition agent called chiral agent to nematic liquid crystal in which rod-like liquid crystal molecules are arrayed in parallel. For example, as illustrated in FIG. 33, cholesteric liquid crystal made by adding addition agent called chiral agent to nematic liquid crystal has a helical structure in which rod-like liquid crystal molecules are twisted.
FIG. 34 is a diagram illustrating a structural example of liquid crystal panels and illustrates a sectional view seen from its lateral side that is obtained by cutting liquid crystal panels in a vertical direction to a liquid crystal display surface. For example, as illustrated in FIG. 34, a liquid crystal display element is created by laminating liquid crystal panels 34-1 to 34-3 into which cholesteric liquid crystal is injected. A liquid crystal display element that uses cholesteric liquid crystal has an excellent characteristic such as a semi-permanent display retention characteristic, a bright color display characteristic, a high contrast ratio, and a high-resolution characteristic.
Moreover, the molecular structure of cholesteric liquid crystal is changed in accordance with the intensity of an applied electric field. For example, when a strong electric field is given to cholesteric liquid crystal, the helical structure of a liquid crystal molecule uncoils perfectly. As a result, the molecular structure of liquid crystal becomes a so-called homeotropic state in which all molecules are arrayed in accordance with the direction of an electric field. Next, when the electric field is suddenly removed from the homeotropic state, the helix axis of the liquid crystal molecule becomes perpendicular to an electrode. As a result, the molecular structure of liquid crystal becomes a so-called planar state in which light according to a pitch of a helical structure is selectively reflected. On the other hand, when a weak electric field by which the helical structure of a liquid crystal molecule is not unfastened is applied to cholesteric liquid crystal and then the electric field is removed, the helix axis of the liquid crystal molecule becomes parallel to an electrode. As a result, the molecular structure of liquid crystal becomes a so-called focal conic state in which incident light is transmitted. Moreover, when a strong electric field is applied to cholesteric liquid crystal and then the electric field is slowly removed or when a medium-size electric field is applied to cholesteric liquid crystal and then the electric field is suddenly removed, a planar state and a focal conic state coexist.
For example, a liquid crystal display element can display a white color in the case of a planar state and can display a black color in the case of a focal conic state. Moreover, in the case of a coexistence state of a planar state and a focal conic state, a liquid crystal display element can display a half tone between white and black. In this manner, a liquid crystal display element that uses cholesteric liquid crystal performs image display by using a phenomenon by which the structure of a liquid crystal molecule is changed in accordance with the intensity of an applied electric field.
Moreover, the liquid crystal display element described above is connected to a common driver that selects a drawing line for drawing an image and a segment driver that outputs a voltage corresponding to a drawing image. The common driver selects drawing lines on the liquid crystal display element one-by-one. When a drawing line is selected by the common driver, the segment driver applies a voltage according to desired image data to be drawn to the drawing line. The structure of a liquid crystal display element that applies voltages to an electrode of the common driver and an electrode of the segment driver and makes liquid crystal display a desired color is referred to as a passive matrix structure.
FIGS. 35 to 37 are diagrams explaining a driving concept of a liquid crystal display element that has a passive matrix structure. FIG. 35 illustrates a driving concept when a first-line image is drawn on a drawing line of a liquid crystal display element. FIG. 36 illustrates a driving concept when second-line image data is drawn on a drawing line of the liquid crystal display element. FIG. 37 illustrates a driving concept when third-line image data is drawn on a drawing line of the liquid crystal display element.
For example, as illustrated in FIG. 35, when the first line of image data is displayed on a liquid crystal display element, a common driver 35-1 selects a first line of a liquid crystal display element 35-3 as a drawing line 35-5 on the basis of selection line data 35-4. Then, a segment driver 35-2 applies a voltage according to first-line image data 35-6 to the drawing line 35-5 selected by the common driver 35-1.
Moreover, as illustrated in FIG. 36, when the second line of image data is displayed on a liquid crystal display element, a common driver 36-1 selects a second line of a liquid crystal display element 36-3 as a drawing line 36-5 on the basis of selection line data 36-4. Then, a segment driver 36-2 applies a voltage according to second-line image data 36-6 to the drawing line 36-5 selected by the common driver 36-1.
For example, as illustrated in FIG. 37, when the third line of image data is displayed on a liquid crystal display element, a common driver 37-1 selects a third line of a liquid crystal display element 37-3 as a drawing line 37-5 on the basis of selection line data 37-4. Then, a segment driver 37-2 applies a voltage according to third-line image data 37-6 to the drawing line 37-5 selected by the common driver 37-1.
As described above, for example, when a desired image is drawn on one drawing line, a common driver applies an uniform pulse voltage to one electrode corresponding to the drawing line among its electrodes. The common driver applies, for example, a 3 V pulse voltage that does not have an influence on the molecular structure of liquid crystal. The common driver plays a role as a switch that selects a drawing line. On the other hand, a segment driver applies a pulse voltage having the size according to desired image data to be drawn to its electrodes. The segment driver applies, for example, a 25 V pulse voltage to an electrode corresponding to a part of which the image data is a black color and applies, for example, a 50 V pulse voltage to an electrode corresponding to a part of which the image data is a white color. Similarly, the whole image data is displayed on a liquid crystal display element by sequentially applying voltages to drawing lines on the liquid crystal display element.
However, along with the large screen of a display device that uses a liquid crystal display element, the completion of an image display requires a long time and thus a new problem is to speed up the complete display of an image. Therefore, although it is considered that the complete display of an image is speeded up by shortening the application time of an alternate-current pulse voltage to be applied to a liquid crystal display element, there is another problem in that the transition of a molecular structure of liquid crystal is not sufficient.
FIGS. 38 and 39 are diagrams illustrating a relationship between a voltage to be applied to a liquid crystal display element and a reflectance of light. Horizontal axes illustrated in FIGS. 38 and 39 indicate an alternate-current pulse voltage to be applied to a liquid crystal display element and vertical axes illustrated in FIGS. 38 and 39 indicate a reflectance of light of the liquid crystal display element.
FIG. 38 illustrates a reflectance of light of a liquid crystal display element when a pulse voltage is applied with a period of 60 milliseconds. Moreover, a reference number 38-1 illustrated in FIG. 38 indicates a situation where the molecular structure of liquid crystal is transited to a planar state, a focal conic state, and a planar state as an applied voltage becomes large. A reference number 38-2 illustrated in FIG. 38 indicates a situation where the molecular structure of liquid crystal is transited from a focal conic state to a planar state as an applied voltage becomes large. FIG. 39 illustrates a reflectance of light of a liquid crystal display element when a pulse voltage is applied with a period of 10 milliseconds. Moreover, a reference number 39-1 illustrated in FIG. 39 indicates a situation where the molecular structure of liquid crystal is transited to a planar state, a focal conic state, and a planar state as an applied voltage becomes large. A reference number 39-2 illustrated in FIG. 39 indicates a situation where the molecular structure of liquid crystal is transited from a focal conic state to a planar state as an applied voltage becomes large.
When comparing FIG. 38 and FIG. 39, it turns out that the molecular structure is not completely transited from a planar state to a focal conic state when a pulse voltage is applied with a period of 10 milliseconds, unlike the case where a pulse voltage is applied with a period of 60 milliseconds. In other words, when a time length for which a pulse voltage is applied to a liquid crystal display element is shortened to speed up the complete display of an image, the transition of a molecular structure of liquid crystal is not sufficient.
Therefore, there is proposed a previous driving method for sufficiently transiting the molecular structure of liquid crystal while planning speeding up of the complete display of an image. The previous driving method is a method for applying a voltage to a certain drawing line and simultaneously pre-applying a voltage having a predetermined size to a line of liquid crystal to be drawn after that, a so-called previous drive line.
FIG. 40 is a diagram explaining a concept of a conventional previous driving method. As illustrated in FIG. 40, in the previous driving method, a voltage is applied to a drawing line 40-1 and simultaneously a voltage is applied to previous drive lines 40-2 that consist of several tens of lines. In other words, in the previous driving method, an energy applied to sufficiently transit the molecular structure of liquid crystal can be given by previously applying a voltage to a previous drive line. The conventional art has been known as disclosed in, for example, International Publication Pamphlet No. WO 2006/103738.
However, the conventional prior driving method described above has a problem in that an image displayed on a liquid crystal display element has unevenness.
FIG. 41 is a diagram illustrating an example of a voltage that is applied to a liquid crystal display element in the conventional prior driving method. Moreover, a “non-selection line” illustrated in FIG. 41 indicates a line other than a drawing line and a previous drive line described above. Moreover, a “planar” illustrated in FIG. 41 means that the molecular structure of liquid crystal is controlled to a planar state. A “focal conic” illustrated in FIG. 41 means that the molecular structure of liquid crystal is controlled to a focal conic state. Moreover, numeric values described in a part corresponding to a reference number “41-1” illustrated in FIG. 41 indicate the values of pulse voltages that are applied from a common driver to a non-selection line, a drawing line, and a previous drive line. Moreover, numeric values described in a part corresponding to a reference number “41-2” illustrated in FIG. 41 indicate the values of pulse voltages that are applied from a segment driver in accordance with a color of an image to be drawn on a drawing line. Moreover, numeric values described in a part corresponding to a reference number “41-3” illustrated in FIG. 41 indicate synthetic values of a pulse voltage that is applied from the segment driver to a segment of the liquid crystal display element and a pulse voltage that is applied from the common driver to a line of the liquid crystal display element. For example, the numeric values of the reference number “41-3” illustrated in FIG. 41 are values that are obtained by subtracting the numeric values of the reference number “41-1” from the numeric values of the reference number “41-2”.
Because a passive matrix structure is a simple lattice structure, a liquid crystal display device having a passive matrix structure has a characteristic that a pulse voltage that is applied to a previous drive line of a liquid crystal display element is the same as a pulse voltage that is applied to a drawing line.
For example, as illustrated by the reference number “41-1” of FIG. 41, the same high-low-mixed pulse voltage is applied from the common driver to the drawing line and the previous drive line. On the other hand, as illustrated by the reference number “41-2” of FIG. 41, a high-low-mixed pulse voltage for controlling the molecular structure of liquid crystal to a planar state or a focal conic state is applied from the segment driver in accordance with a color tone of an image to be drawn on the drawing line.
Then, as illustrated in FIG. 41, the pulse voltage applied to the previous drive line by the segment driver becomes the same as the pulse voltage applied to the drawing line to be a high-low-mixed voltage. For example, as illustrated by “41-4” and “41-5” of FIG. 41, when the molecular structure of liquid crystal is controlled to a planar state, the pulse voltages applied to the previous drive line and the drawing line from the segment driver have the same size. Similarly, as illustrated by “41-6” and “41-7” of FIG. 41, when the molecular structure of liquid crystal is controlled to a focal conic state, the pulse voltages applied to the previous drive line and the drawing line from the segment driver have the same size.
As described above, when a certain line of previous drive lines is a drawing target, the certain line is affected by the pre-applied pulse voltage because the same pulse voltage as that of a drawing line is applied to a previous drive line. For this reason, it can be considered that the molecular structure of liquid crystal is not sufficiently transited depending on the size of a pulse voltage applied to the previous drive line. Therefore, this consequently leads to display an uneven image on a liquid crystal display element.
FIG. 42 is a diagram illustrating an example of an uneven image that is displayed on a liquid crystal display element. As illustrated in FIG. 42, when a liquid crystal display element 42-1 is drawn, for example, in a direction of 42-4, unevenness occurs like the case where a portion 42-2 to be originally displayed with a black color becomes slightly white or like the case where a portion 42-3 to be originally displayed with a white color becomes slightly black.
For example, although it is preferable to transit the molecular structure of liquid crystal to a focal conic state when an image is drawn on a drawing line in a black color, a certain level of a voltage application time is spent to transit to the focal conic state as described above. However, because the transition of a molecular structure of liquid crystal corresponding to a previous drive line is dependent on the size of a voltage applied to a drawing line, the molecular structure may not be sufficiently transited to a focal conic state in some cases. In this case, a difference occurs between the brightness of black color images drawn on the drawing line, and thus a portion such as the portion 42-2 illustrated in FIG. 42 occurs.
For example, although it is preferable to transit the molecular structure of liquid crystal to a planar state when an image is drawn on a drawing line in a white color, a certain level of field intensity is applied to transit to the planar state as described above. However, because the transition of a molecular structure of liquid crystal corresponding to a previous drive line is dependent on the size of a voltage applied to a drawing line, the molecular structure may not be sufficiently transited to a planar state in some cases. In this case, a difference occurs between the brightness of white color images drawn on the drawing line, and thus a portion such as the portion 42-3 illustrated in FIG. 42 occurs.
Moreover, although an uniform voltage can be applied to a previous drive line when an active matrix structure having a switch element is applied to a liquid crystal display element, this is not preferable from the viewpoint of controllability and cost because the structure is complicated.